The present invention relates to transferring massive loads between objects and, more particularly, to such a transfer during relative movement between objects.
Off-shore well platforms are commonly used to extract crude oil, natural gas and/or other hydrocarbons from the earth beneath a body of water. Such platforms are relatively massive structures designed often to support not only the equipment for the actual drilling or extraction but also living quarters for a crew, hydrocarbon processing equipment, helicopter landing pads, cranes for transferring supplies and other material between the same and cargo ships, etc.
Each platform generally is made up of two major parts, one of which is a deck which supports the derrick and other components desired to be maintained at the surface of the body of the water. The other major part is a base which supports the deck and is either directly connected to, or tethered to, the earth at the platform location. A fixed structure providing a base typically is referred to in the art as a jacket, whereas one that floats and is tethered referred to as a hull.
The base of an off-shore platform generally is made first and the deck then assembled to the same in parts or modules. This construction is both time consuming and quite complex. Part of this is due to the waves and currents in the body of water. It will be seen that the uncontrolled motion represented by the same can interfere greatly with the transfer of the deck modules to the base. While such uncontrolled motion is generally cyclical in nature, it is sufficiently random that the amount of potential dynamic loading which might be experienced between a deck module, a deck module carrier (floating barge) and/or the base during transfer has severely restricted the mass of deck modules which can be transferred. (While as stated the mass of possible deck modules is severely restricted, this is relative--deck modules having a weight of over 9,000 tons are transferred by prior art techniques.) This random motion includes both a vertical component and a lateral component.
The problem is particularly acute when the base itself is subjected to wind, wave or current motion. In this connection, because of the difficulty associated with constructing a platform at the extraction or, in other words, the production location, in some instances in which there is a calm body of water in direct communication with the final location desired for the platform, e.g., a Norwegian fjord and the North Sea, it is common to assemble the same as a floating structure in the calm body of water, then to tow the platform to the borehole site to be tethered. It should be mentioned that some of these calm bodies of water, such as many Norwegian fjords, are calmer than lakes of a comparable size, and this "preassembly" technique is often preferred if it is practical.
In efforts to overcome the construction hurdle associated with water motion, those in the art have used large separate crane ships to transfer deck modules to bases. Such crane arrangements are even used in relatively calm bodies of water in order to increase the size of deck modules which can be assembled on a towable base. Even when a crane arrangement is used, it is necessary to include large shock absorbing structures, such as bulk elastomers, crushable foams, aluminum cylinders (in essence, aluminum "culverts"), sand or hydraulic systems, to prevent damage due to high impact loads. Floating cranes that are suitable are very costly, limited in capacity, and only can be used during relatively calm sea states.
As off-shore platforms become larger and heavier, the problem of being limited in the size of the deck modules which can be transferred to bases becomes even more of a limitation. Heavier decks, or deck modules, have greater masses, and higher dynamic loads are potentially experienced because of the relative motion. Also, larger decks often have major overhangs which amplify the relative motion at their outboard edges and the resulting load which can be experienced at the deck support points.
Significant resources have been spent in an effort to overcome the construction hurdle caused by water motion. It has been proposed that a barge or the like carrying a deck module be maneuvered between the supporting structures of a base so that parts of the module are above the supports. The barge is then lowered in the water, with the result that the deck module itself is "transferred" to the base. An example of such an arrangement is that described in U.S. Pat. No. 4,662,788. A difficulty in the past, though, is that arrangements which have been designed have not taken into account the lateral (linear and torsional) shear loading which is caused by relative motion between the carrier and the base during the transfer. It will be obvious, for example, that the stabbing pins included in the arrangement disclosed in the above-mentioned patent, cannot withstand any significant shear loading. While lateral motion can be minimized by tethering and the like, it has been found that as a practical matter it cannot be eliminated altogether. The failure of many to take shear loading into consideration is not altogether surprising in that the desired transfer motion is only longitudinal.
One other problem which has prevented this technique from being used in any situation other than one in which the waters are quite calm, is the risk that the relative motion during the transfer may cause the carrier to strike or reengage the deck module after initial disengagement but before the transfer is completed and/or the carrier is moved away. Again, prior arrangements have not taken this possibility into consideration.